17. The following appeared in an article in a consumer-products magazine.
“Two of today’s best-selling brands of full-strength prescription¹ medication for the relief of excess stomach acid, Acid-Ease and Pepticaid, are now available in milder nonprescription forms. Doctors have written 76 million more prescriptions² for full-strength Acid-Ease than for full-strength Pepticaid. So people who need an effective but milder nonprescription medication for the relief of excess stomach acid should choose Acid-Ease.”
Discuss how well reasoned... etc.
This ad recommends non-prescription Acid-Ease over non-prescription Pepticaid for relief of excess stomach acid. The only reason offered is that doctors have written 76 million more prescriptions for the full-strength prescription form of Acid-Ease than for full-strength Pepticaid. While this reason is relevant, and provides some grounds for preferring Acid-Ease over Pepticaid, it is insufficient³ as it stands because it depends on three unwarranted assumptions.
The first assumption is that the prescription form of Acid-Ease is more popular among doctors. But this might not be the case, even though doctors have written 76 million more prescriptions for Acid-Ease. Acid-Ease may have been available for several more years than Pepticaid; and in the years when both products were available, Pepticaid might have actually been prescribed more often than Acid-Ease.
The second assumption is that doctors prefer the prescription form of Acid-Ease for the reason that it is in fact more effective at relieving excess stomach acid. However, doctors may have preferred Acid-Ease for reasons other than its effectiveness. Perhaps Acid-Ease is produced by a larger, more familiar drug company or by one that distributes more free samples. For that matter, the medical community may have simply been mistaken in thinking that Acid-Ease was more effective. In short, the number of prescriptions by itself is not conclusive⁴ as to whether one product is actually better than another.
The third assumption is that the milder non-prescription forms of Acid-Ease and Pepticaid will be analogous⁵ to the full-strength prescription forms of each. But this might not be the case. Suppose for the moment that the greater effectiveness of prescription Acid-Ease has been established; even so, the non-prescription form might not measure up to non-prescription Pepticaid. This fact must be established independently.
In conclusion, this ad does not provide enough support for its recommending non-prescription Acid-Ease over non-prescription Pepticaid. To strengthen its argument, the promoter of Acid-Ease would have to show that (1) the comparison between the number of prescriptions is based on the same time period; (2) its effectiveness is the main reason more doctors have prescribed it, and (3) the comparative effectiveness of the two non-prescription forms is analogous to that of the prescription forms.

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